The overall objectives of this proposal are to characterize in vitro erythropoietic regulation by quantitating erythropoietin (ESF)-responsive compartments in normal animals and man. The study will investigate those factors influencing compartment size and cell cycle characteristics -including transfusion suppression and hypoxic stimulation of erythropoiesis. The major methods employed will include colony assays for both early (erythroid burst forming cells) and late (erythroid colony forming units) ESF-responsive cells, as well as techniques to analyze their physical and cell cycle characteristics. The flow from early, less ESF-sensitive populations, to later, more responsive erythroid cells will be characterized. Results in experimental animals will be compared to normal man and to erythropoietic regulation in polycythemia vera. In addition, studies to better define the effect of steroid compounds on erythropoiesis will be carried out, including the possible value of in vitro studies in predicting the clinical response to these drugs of patients with marrow failure. Finally, techniques will be explored to permit the in vitro assay of ESF using marrow cells in liquid culture.